In digital communications, it is necessary to modulate binary information and to transmit it over a communication channel. This channel may be a hard-wired channel or a radio channel.
One method of modulating binary information is frequency shift keying (FSK). In FSK, the carrier frequency is switched or keyed between two values of frequency. One application in which FSK is used is caller identification (CID) services provided by telephone companies. The CID services deliver information such as a caller's telephone number and/or name to a called subscriber's telephone at the beginning of a call. In most countries, the caller identity data stream is transmitted in 1.2 Kbps Bell 202 standard or CCITT V0.23 FSK format. In such FSK format, a one "1" is represented by a first frequency (e.g., 1.2 KHz) of a carrier signal, which is typically a sinusoidal waveform, and a zero "0" is represented by a second frequency (e.g., 2.2 KHz) of the carrier signal. The FSK signal is transmitted via a hard-wired channel (i.e., telephone line) to a telephone set of a called subscriber. The transmitted signal has two frequencies reflecting the binary information to be transmitted over time. The called subscriber's telephone set demodulates the FSK-modulated signal and displays the CID information on a display device such as an LCD or the like.
Such demodulators for CID (caller identification) systems are typically implemented by analog-to-digital (A/D) converters. The A/D converters contribute to the increased demodulator chip size and complexity of the hardware.